Metals and alloys will undergo an expansion in size when subjected to elevated temperatures. The degree of this expansion is characterized by the material property known as the coefficient of thermal expansion (COTE). The COTE is a function of both material properties (composition, thermal history, etc.) and external variables (most notably the temperature). The COTE of an alloy is a key property considered in the design of components in most types of mechanical systems operating at elevated temperatures.
Low thermal expansion alloys have been employed in gas turbine engines to provide a high level of dimensional control in critical components such as seal and containment rings, cases, and fasteners. In such applications, other important properties can include mechanical strength, containment capabilities, and oxidation resistance. One alloy which possesses such properties is HAYNES® 242® alloy, developed, manufactured, and sold by Haynes International. This is a Ni—Mo—Cr alloy with a nominal composition of Ni-25Mo-8Cr (all compositions in this document are given in wt. % unless otherwise noted). This alloy was covered by U.S. Pat. No. 4,818,486 of Michael F. Rothman and Hani M. Tawancy which was assigned to Haynes International Inc. The 242 alloy is currently employed in numerous gas turbine applications in both the aero and land-based gas turbine industries.
HAYNES 242 alloy is a high strength, low COTE alloy designed for use in gas turbine engines. It is strengthened by an age-hardening heat treatment which results in the formation of long range ordered domains of the Ni2 (Mo, Cr) phase. These domains provide high tensile and creep strength at temperatures up to around 1300° F. (704° C.). The COTE of 242 alloy is low compared to other Ni-base alloys. This can be attributed to the presence of a high molybdenum (Mo) content in the alloy (25 wt. %). Mo is well known to lower the COTE of nickel-base alloys. Another key feature of 242 alloy is the good oxidation resistance. The presence of 8 wt. % Cr provides sufficient oxidation resistance for use without a protective coating being necessary or in applications where some measure of oxidation resistance is desirable in the event of spallation of the protective coating. Yet another key feature of 242 alloy is its excellent fabricability (formability, hot/cold workability, and weldability) with respect to other age-hardenable nickel-base alloys. Ni-base alloys which are age-hardenable by the gamma-prime phase, for example, are well known to be susceptible to fabrication issues, arising from the fast precipitation kinetics of the gamma-prime phase. In contrast, the Ni2 (Mo, Cr) phase responsible for age-hardening in 242 alloy has slow precipitation kinetics and therefore 242 alloy does not suffer from the fabricability problems described above.
However, the maximum use temperature of age-hardened 242 alloy (around 1200 to 1300° F./(649 to 704° C.)) can limit the use of the alloy in certain applications. As designers are pushing the operating temperatures to higher and higher levels, the need for a low COTE alloy capable of operating at higher temperatures is becoming necessary. A low COTE alloy which can maintain its high mechanical strength to temperatures of 1400° F. (760° C.) or more would represent a significant advantage to the gas turbine industry.